In this context, the term “precious metal” includes platinum, gold, rhenium, all other metals in the platinum group, and the alloys of the aforementioned metals, and the stated metals and alloys in oxide dispersion-strengthened form. Molybdenum is used in particular as the refractory metal, as are tungsten, niobium and tantalum.
It is known that, at high temperatures such as those that occur in a glass melt, for example, a small portion of the water that is present breaks down into hydrogen and oxygen. When the glass melt comes in contact with components made of precious metals, in particular platinum and its alloys, the hydrogen that is formed can pass through the platinum part. As a result, the oxygen in the melt is enriched and oxygen bubbles form that—provided no further action is carried out—remain in the finished glass product and lower its quality in a critical manner. Particularly problematic in this regard is the fact that precious metal components are used primarily directly after the refining area, making it very difficult to remove the oxygen bubbles that form on the precious metal components from the glass melt.
Publication EP 1 101 740 A1 therefore proposes that bubbles be prevented from forming by applying a reverse voltage via electrochemical means. A variant is proposed, among others, with which the one molybdenum electrode is located in the glass melt upstream of the precious metal components, the electrode being connected in a conductive manner with the precious metal components. An adequate reverse voltage is generated as a result without an external power supply. A method is described in publication U.S. Pat. No. 5,785,726, with which oxygen bubbles are prevented from forming in the glass melt by preventing hydrogen from the glass melt from escaping through the precious metal walls by establishing an atmosphere on the side of the precious metal walls facing away from the glass melt that contains a high percentage of water vapour. The purpose of this is to prevent hydrogen gas from the glass melt from diffusing to the outside through the precious metal walls made permeable by the high temperatures. With the two known methods, oxygen bubbles can indeed be largely prevented from forming in the glass melt on precious metal components, but, as a result, the oxygen partial pressure is held at the level that existed before the melt flowed into the platinum system.
It has been shown that bubbles still form with the known procedures described above, however, and defective glasses are produced as a result.
The problem addressed by the present invention is to provide a method and a device for producing glass, with which the formation of bubbles on precious metal components—and other disturbances—can be reliably prevented.